1. Field of the Invention
The present invention relates to a compound having an unsaturated side chain, a process for preparing the same, a liquid crystalline polymer prepared from the same, a liquid crystal mixture and a display element.
2. Description of the Related Art
In these days, as a liquid crystal display, a twisted nematic (TN) liquid crystal display is most widely used. A TN liquid crystal display has various advantages that a low driving voltage, a low power demand, and the like. However, its response speed is slower than a light-emitting type display element such as a cathode ray tube display, an electroluminescence display, a plasma display, etc.
A super twisted nematic (STN) liquid crystal display having an increased twist angle from 180.degree. C. to 270.degree. C. has been developed, but its response speed is still insufficient.
Though various improvements have been made in the liquid crystal displays, no TN liquid crystal display having a high response speed has been developed.
In a new display using a ferroelectric liquid crystal which is recently vigorously studied, a response speed may be expected to be increased greatly (see Clark et al, Appl. Phys. Lett., 36, 899 (1980)). This type of display makes use of a chiral smectic phase such as a chiral smectic C phase (hereinafter referred to as a "Sc* phase") which has a ferroelectric property. As phases having the ferroelectric property, there are known chiral smectic F, G, H and I phases in addition to the Sc* phase.
The ferroelectric liquid crystal material which is actually used in a ferroelectric liquid crystal element is required to have various characteristics. But, at present, a single compound cannot satisfy all the characteristics, and it is necessary to formulate a ferroelectric liquid crystal mixture comprising plural liquid crystal compounds, or at least one liquid crystal compound and at least one non-liquid crystal compound.
In addition to the ferroelectric liquid crystal mixture consisting of the ferroelectric liquid crystal compounds, Japanese Patent KOKAI Publication No. 195187/1986 discloses the preparation of a ferroelectric liquid crystal mixture by mixing a compound having a nonchiral smectic C, F, G, H or I phase (hereinafter referred to as "Sc-like phase") or a mixture containing such compound with at least one compound having the ferroelectric phase to make the liquid crystal mixture ferroelectric as a whole.
Mol. Cryst. Liq. Cryst., 89, 327 (1982) provides a ferroelectric liquid crystal mixture comprising a compound or a mixture having the Sc-like phase and at least one compound which is optically active but has no ferroelectric phase.
From the above, it is understood that a ferroelectric liquid crystal mixture can be formulated from at least one optically active compound which may have the ferroelectric phase or not, as a base compound. Preferably, the optically active compound has a liquid crystal phase. When it has no liquid crystal phase, it is preferred that its structure is similar to a liquid crystal compound, namely it is a quasi-liquid crystal compound.
However, there has been found no liquid crystal substance which has spontaneous polarization required for the high speed response and a low viscosity and exhibits the ferroelectric phase in a wide temperature range including room temperature.
Further, no report has been made on the use of such liquid crystal substance as a monomer component of a polymer.
Comparing a display element using a low molecular weight liquid crystal which is widely used in an electronic calculator, a watch or a display and a display element using a liquid crystalline polymer, the former has the following drawbacks. That is, as a display using the low molecular weight liquid crystal, there is used a panel comprising a pair of glass plates a gap of which is controlled in the order of several micrometers and the low molecular weight liquid crystal is interposed between the glass plates. In this case, it is extremely difficult to control the gap between the glass plates in the order of several micrometers over a wide area, such display has poor impact resistance, and it is difficult to constantly supply the wide area displays due to the influence of gravity. On the other hand, since the liquid crystalline polymer is a macromolecule, it has a self-supporting property, so that it may overcome the drawbacks of the low molecular weight liquid crystal display (see J. Polym. Sci., Polym. Lett., Ed. 13, 243 (1975) and Polym. Bull., 6, 309 (1982)).
One of the drawbacks of the liquid crystalline polymer is a long response time against a change of an external factor such as an electric field, so that it cannot be used to display moving pictures.